PHNOM PENH, Cambodia — Disputes in the strategically important South China Sea proved so contentious here on Thursday that an annual regional gathering ended without even a basic diplomatic communiqué, which appeared to have been blocked by China.

The host for the conference of the Association of Southeast Asian Nations, Cambodia, a close ally of China, refused to play the customary role of seeking agreement among the 10 participating countries, thus undermining the possibility of an accord, according to a senior diplomat from the association.

“China bought the chair, simple as that,” said the diplomat, who declined to be identified publicly according to usual protocol. The diplomat pointed to an article on Thursday by China’s state news agency, Xinhua, in which the country’s foreign minister, Yang Jiechi, was quoted as thanking Cambodia’s prime minister for supporting China’s “core interests.”

Secretary of State Hillary Rodham Clinton, who met with foreign ministers at the conference of the Southeast Asian group, said in remarks clearly aimed at China that it was important that the disputes be resolved “without coercion, without intimidation, without threats and without use of force.”

The influence of China, which was represented here by Mr. Yang, hung over the behind-the-scenes deliberations on the South China Sea in many respects, dividing countries that are beholden to China and those that are willing to stand up to the Chinese.

Cambodia receives large amounts of assistance from Beijing, including new military aid that it got a few months ago.

Indonesia, which has no territorial claims in the South China Sea, tried to forge a last-minute consensus at the meeting but without success. The Indonesian foreign minister, Marty Natalegawa, praised Mrs. Clinton for “showing interest but giving space” in the effort to reach an agreement.

Territorial disputes in the South China Sea have increased in the last several months between the Philippines and China, and between Vietnam and China. One conflict, which lasted for months, involved a standoff between lightly armed vessels belonging to China and to the Philippines at the Scarborough Shoal off the coast of the Philippines. Another dispute centered on a law enacted in Vietnam claiming sovereignty over the Paracel and Spratly Islands, which China also claims.

As the long-dominant United States and the fast-growing China both seek to increase their naval power in the Asia-Pacific region, the disputes have become more threatening.

China has repeatedly told American diplomats that the energy-rich South China Sea is none of Washington’s business.

But the Obama administration has made clear that freedom of navigation is at stake in one of the world’s most important bodies of water for commerce.

In her statement to the news media, Mrs. Clinton said, “The United States is a resident Pacific power,” a term intended to signal to China and the countries of the region that the United States is staying, and even increasing its presence.

“No nation can fail to be concerned by the increase in tensions, the uptick in confrontational rhetoric and disagreement over resource exploitation,” Mrs. Clinton said.

“We have seen worrisome instances of economic coercion and the problematic use of military and government vessels in connection with disputes among fishermen,” she said. The mention of economic coercion appeared to be a reference to China’s decision to stop importing Philippine bananas and to clamp down on Chinese tourist groups.

China has made clear that it wants to deal with the South China Sea disputes with each country individually, and not through any regional forum. That stance has made the future of a code of conduct to resolve disputes in the South China Sea unclear.

Asian diplomats said Thursday that the main elements of a code of conduct, which the United States has urged the Southeast Asian group to adopt, had been agreed upon at this week’s gathering. The diplomats declined to specify the content of the proposed code.

The spokesman for China’s Ministry of Foreign Affairs has repeatedly said that China is willing to discuss a code of conduct only “when conditions mature.”

Last Sunday, Mrs. Clinton began a tour of Asia that is intended to show that the administration’s shift to that region reaches beyond military engagement.

The trip drew negative coverage in the Chinese press on Thursday. People’s Daily wrote that a trade agreement that Washington is seeking, called the Trans-Pacific Partnership, which excludes China, was an effort to weaken Asian integration. China Business News referred to “those hyping up the South China Sea issue,” a veiled reference to the United States.

The meeting here was held in a white-pillared conference hall, called the Peace Palace, that was built for the occasion by the Chinese government. When a Cambodian reporter asked Mrs. Clinton about American assistance to Cambodia, she made a reference to the difference between Chinese aid and that provided by the United States.

“We can’t point to a big building,” she said, indicating that American aid was directed at feeding people in need, ensuring the survival of women who give birth and trying to improve people’s lives, especially those of children.